This invention relates generally to gas turbine engines, and more specifically to turbine casings used with gas turbine engines.
Gas turbine engines generally include, in serial flow arrangement, a high pressure compressor for compressing air flowing through the engine, a combustor in which fuel is mixed with the compressed air and ignited to form a high energy gas stream, and a high pressure turbine. The high pressure compressor, combustor and high pressure turbine are sometimes collectively referred to as the core engine. Such gas turbine engines also may include a low pressure compressor, or booster, for supplying compressed air to the high pressure compressor.
At least some known turbines include a rotor assembly including a plurality of rows of rotor blades. Each rotor blade extends radially outward from a blade platform to a tip. A plurality of shrouds couple together to form a flow path casing that extends substantially circumferentially around the rotor assembly, such that a tip clearance is defined between each respective rotor blade tip and the casing. The tip clearance is designed to be a minimum, while still being sized large enough to facilitate rub-free engine operation through a range of available engine operating conditions.
During operation, turbine performance may be influenced by the tip clearance between turbine blade tips and the shroud. Specifically, as the clearance increases, leakage across the rotor blade tips may adversely limit the performance of the turbine assembly. To facilitate maintaining blade tip clearance at least some known shroud designs attempt to match the rate of thermal expansion of the stator case to the rate of thermal expansion of the turbine rotor assembly by supplying a variable amount of cooling fan air to the casing flanges. Cooling the flanges facilitates controlling thermal movement to facilitate eliminating rocking of the shrouds. The mass at the flange also pushes the casing downward to facilitate maintaining blade tip clearances.
To facilitate the controlling of thermal movement and the maintaining of blade tip clearances, casing members include a pseudo flange which adds structural integrity to the shroud casing.
In some instances, the pseudo flange is hourglass-shaped with a large mass of material formed at its outer diameter and a thin mid section. However, fabricating such pseudo flanges may be both expensive and time consuming.